1. Field of the Invention
The present invention relates to an absolute position length-measurement type encoder.
2. Description of the Related Art
Incremental encoders and absolute encoders are known as devices for measuring travel distances of objects. The incremental encoders measure relative travel distances and absolute encoders allow for absolute position length-measurement.
In the case of photoelectric encoders, the incremental encoders have incremental tracks with incremental patterns including equally-spaced light and dark patterns. Based on these patterns, the incremental encoders count light and dark signals to detect relative travel distances. In addition, the incremental encoders may detect absolute travel distances by detecting origin detection patterns provided separately from the above-mentioned pattern with equally-spaced light and dark patterns, and then detecting relative travel distances from the origin. However, prior to the measurement, a scale must be moved to right and/or left directions in order to read origin detection patterns.
On the other hand, the absolute encoders have absolute tracks with absolute patterns representing pseudo-random codes such as M-sequence codes and detect absolute positions resulting from reading the absolute patterns for a corresponding object. Unlike the incremental encoders, the absolute encoders does not require any origin detection based on origin detection patterns and may start measurement from the very position when powered on, without moving the scale. However, the absolute encoders have a lower detection accuracy than the incremental encoders.
As such, an absolute position length-measurement type encoder is known where an incremental track with equally-spaced incremental patterns and an absolute track with absolute patterns representing pseudo-random codes are positioned in parallel on one scale, as disclosed in, e.g., JP H7-286861A. This encoder first detects the absolute position after powered on by reading absolute patterns on the absolute track. Then, the encoder detects a relative travel distance from that position by reading the incremental patterns on the incremental track. In this way, an absolute position length-measurement type encoder may be obtained that covers the shortcomings of each of the incremental and absolute encoders, while enjoying advantages of both encoders.
However, in encoders so configured, it is more difficult to form minute absolute patterns with respect to incremental patterns without positional errors, as incremental patterns have more minute light and dark pitches. In addition, as the entire length of a scale becomes longer, it is more difficult to maintain a relative phase relation between absolute patterns and incremental patterns throughout the scale.
Therefore, it is difficult to provide smaller absolute position length-measurement type encoders in which both absolute and incremental patterns are used.